I bought my copy used and it has a bunch of lap records that are at 0:00 or insanely unbeatable times. I figure this must have been done using the cheats. I've erased all save files but these records remain. This is really annoying.

To erase the lap records on the N64 version of the game, first erase all the saved players by selecting them on the Player Select screen then pressing the C up button. Once all the player slots are empty, you can select the "Practice" slot and press C up to delete all the other stored data in the cartridge. All tracks and characters that were unlocked will be locked again and all lap times will be cleared so the cartridge will be the same as when it came from the factory.